Liquid level electro-optical sensing devices are well known. They operate on the principle of different indices of refraction between the transparent body of a probe and a gaseous medium above the liquid, normally air. Such probe bodies are provided with an optically polished exterior surface area upon wich a light beam is shined from the interior of the body. When the surface area is exposed to the gaseous medium, i.e., air, the light beam is reflected from the surface area interiorly of the body and the reflection shines upon a receiving electro-optical transducer to develop an electrical signal. On the other hand, when the reflecting area is immersed in the liquid, the light beam exits the body through the area without appreciable reflection so that the electro-optical transducer develops no electrical signal. The presence or absence of such a signal indicates whether the probe reflecting area is dry or wet, i.e. immersed in the liquid. Such devices are used as high liquid level alarms and low liquid level alarms. When used as a high liquid level alarm, the probe surface reflecting area normally is not immersed in the liquid, and when used as a low liquid level alarm, usually is immersed in the liquid.
When the probe is used as a high liquid alarm, the reflecting area normally is dry so that the light circuit to the receiving transducer normally is completed and an electrical signal normally is developed. This enables the system to be readily self-checked or self-monitored, and to be fail-safe, because interruption of the signal while the probe surface area is dry, i.e., its normal state, will indicate that one or more elements of the system are not functioning properly. On the other hand, when such a device is used as a low liquid level alarm so that the reflecting area is normally immersed in the liquid, and consequently the light circuit normally is interrupted because the light beam exits from the body, the system is not readily susceptible to self-checking or self-monitoring and is not normally fail-safe because the electro-optical transducer does not normally develop an electrical signal. Thus, the failure of any one of the elements of the system interrupts no signal.
Solutions to the problem of making high liquid level electro-optical sensing device, self-checking or fail-safe under any conditions are known. For example, the patent to Perren, U.S. Pat. No. 4,201,914, May 6, 1980, shows an electro-optical liquid level sensing device which employs an infrared source that is cyclically-modulated in a self-monitoring mode to cause the system to oscillate. Another patent to Jacobsen, U.S. Pat. No. 4,134,022, Jan. 9, 1979, discloses a comparable fail-safe device which uses a light source of a predetermined frequency. The disclosures of both of these patents, however, relate to high liquid level sensing devices where the problem of self-monitoring under normal conditions does not exist.